Analytical and numerical analysis of iceberg collisions with ship structures

The scope of the present Ph.D. thesis was to investigate the mechanics of collisions between icebergs and ship structures. Large reservoirs of oil and gas and new sailing routes in the Arctic area are two large motivations for the present research. Human activity in the Arctic demands well-designed naval architectures for facing different kinds of ice loads. According to modern design codes, ship and offshore structures are usually designed under the ultimate limit state (ULS) methods and checked with the accidental limit state (ALS) methods. The methods and assumptions presented in this thesis are only valid for ALS design. This thesis considers three topics related to the iceberg collisions with ship structures: external mechanics, internal mechanics and residual strength assessments. Collisions between icebergs and ship structures are quite complicated processes. For simplicity, this problem was split into its external and internal mechanics. External mechanics talks about the translational and rotational momentum balance. It is believed that the impact mechanics of iceberg collisions with ship structures should be presented in three dimensions. Thus, a new formulation of impact mechanics was proposed that describes the impact in three dimensions. All forces except for impact force are neglected. This new three-dimensional method degrades to existing two dimension (2D) method. It was successfully applied to the calculation of the demanded dissipated energy for iceberg and ship collisions. Internal mechanics deal with both deformations of icebergs and ship structures. Nonlinear finite-element analysis (NLFEA) was used in this research. The commercial code LS-DYNA 971 was used to assess the internal mechanics of both icebergs and ship structures. Deformations of both icebergs and ship structures should be well captured by numerical simulations. However, due to the difficulties of simulating ice, NLFEA is not straightforward. To facilitate such simulations, a plasticity-based material model for icebergs was developed in this thesis. Iceberg crack propagation was simulated by element erosion. An empirical failure criterion for detecting those failed ice elements is proposed. Numerical examples showed that the new iceberg model gives good results. The model was successfully implemented in LS-DYNA 971 through a user-defined subroutine. Subsequently, the integrated numerical analysis of iceberg-ship collisions was then successfully performed. Efforts were made to investigate the internal mechanics of both icebergs and ship structures during collision, such as local structural behaviours and ice failure. Two scenarios of iceberg-ship collisions were investigated: iceberg collisions with foreship and side-ship structures. In the first scenario, efforts focused on the investigation of the detailed internal mechanics of the icebergs and ship structures. The strength of ship structures was varied by adjusting the parameters of the steel material model, thereby varying the relative strength of icebergs and ship structures. A comprehensive discussion is based on the simulation results. The discussion addresses contact pressure, iceberg shapes and collision locations. In the second scenario, investigations focused on the influences of the iceberg shapes. Simple iceberg shapes representing “sharp” and “blunt” icebergs were used. The results show that “blunt” icebergs may behave as rigid bodies. Finally, the residual strengths of the ship structures after impact, which may be caused by the icebergs, was assessed. A simple plasticity method and a single stiffener model were developed to quantify the residual strengths of the damaged ship structures. In the simple plasticity method, elastic and rigid-plastic methods were combined to derive the end-shortening curve for damaged stiffeners. In the single stiffener model, proper boundary conditions were proposed. Both methods were verified against numerical simulations. Generally, good results were obtained. From this work, a rapid method to assess the residual strength of damaged ship structures is suggested.PhD i marin teknikkPhD in Marine Technolog